To be perfectly clear - while it does happen I would never recommend GOING on a cruise (as the OP title suggests) in order to hook up. Most cruises are couples and familie traveling together, and a single will often have a hard time finding other singles. Now, once you find them it will likely be late at night in the disco as their parents or kids are asleep. So, you might "get lucky" but don't expect a lot of cuddling the next day, because there is a good chance you won't see them again until the next night. My point is, it is a little like going to DisneyWorld to hook up - plenty of people, but not the right kind.
The exception is singles group cruises. there are plenty of singles curise operators who put together groups, matching up same sex roommates, and then making sure there are plenty of singles and activities daly to get them mingling. THIS is a good bet for "hooking up" or just meeting someone nice, or at the least having companionship on a cruise instead of being alone.
How would I know all this? I worked on cruise ships for 2 years (like a poster above, I was a JAR, and he will know what I mean). And today I am the editor of cruisemates.com - one of the leading online cruise guides. We have a large section just for singles cruisers (articles and message boards) if you want to research this further.